The present invention relates to railroad car hot box detectors and more particularly to a unique system for treating the heat signal generated by such detectors.
In order to protect against railroad car wheel bearing failure, most railroads utilize hot box detectors along their rights of way to view, through infra-red scanners, the bearings of railroad cars as they pass through a sensing zone. If an overheated bearing is detected, some type of alarm is triggered to alert the engineer to stop the train and correct the potentially dangerous situation which, if allowed to continue, could result in a train derailment. While it is extremely important that no overheated bearings (i.e., hot boxes) be missed by the hot box detector, it is almost equally important that no false alarms be generated since the unscheduled stopping of a train is a costly and time consuming operation that could result in substantial disruptions of schedules.
The infra-red scanner and associated circuits for detecting overheated bearings are highly developed and available commercially from such sources as the Servo Corporation of America of Hicksville N.Y. The equipment is disclosed, for example in U.S. Pat. Nos. 3,545,005; 3,454,758; 3,812,343; 3,872,456 and 4,113,211.
Heretofore hot box detector systems of the type described in the above patents and those available commercially have been designed to safely detect hot boxes for trains passing a scanning site under a wide variety of different conditions without regard to the specific train conditions. Thus, the systems described above are designed so that they operate to process signals the same way whether the signal is generated from a train proceeding along at five miles an hour or from a high speed train moving at speeds that can exceed 100 miles per hour.
The hot box detector scanner, in effect produces a scanning spot along an optical axis which is positioned to image an area through which the train bearing is expected to pass. As a result of the train moving past the scanner, the train velocity and the scanning spot inter-relate to convert the continuous spatial heat analog information of the train into continuous analog signals in the time domain. The product of train velocity in inches per second and the scanner spatial resolution per inch give the equivalent system sampling rate in time.
It has been determined that one-half the described product defines the minimum system bandwidth required to convert the spatial harmonics resolved by the scanning spot into temporal harmonics. In other words, the system bandwidth, within limits is a direct function of the speed of the passing train and the scanning spot size of the infra-red detector. However, for any particular site the scanning spot is fixed and thus the bandwidth is a function of the speed of the passing train. Typical nominal values of interest are shown in the table below:
______________________________________ Minimum System Train velocity Scanning Rate Bandwidth MPH Inches/Sec Spots/Sec Hz ______________________________________ 5 88 61.6 30.8 13 228.6 160 80 48.7 857 600 300 60 1056 739.2 369.5 80 1408 985.6 492.8 150 2640 1848 924 ______________________________________
From the above, it can readily be appreciated that the use of a fixed system bandwidth equal to that needed to avoid attentuation of the harmonics at the fastest anticipated train speed (i.e., 924 Hz for 150 MPH) places the system at an extreme disadvantage when the train is passing the scanning zone at lower valocities that do not need extra bandwidth since any noise with components in the extra bandwidth will be amplified and will contaminate the data obtained by the scanner.
In practice, commercial hot box detector systems are signal bandwidth limited to a constant value of about 300 Hz because a wider bandwidth allows the amplification of intolerable noise. From the above table, it can been seen that this bandwidth corresponds to a train speed of approximately 50 miles per hour. Any train passing the scanner at a velocity greater than 50 miles per hour will generate a signal with attenuated harmonics as a result of which the shape, including the peak value of the heat signal is distorted by the attenuation and the absence of the higher harmonics.
A somewhat analogous situation arises in the manner in which the value of the heat signals are treated. A hot box detector system looks for abnormal heat build up in bearings which would occur if the bearing lubricant failed or any other mechanical failure occurred. Hot box detector systems presently available treat the bearing heat signal as a value above a reference signal which may, for example, be generated by a reference heat signal source built into the system. What is important, is the rise of the bearing temperature over the general temperature of the passing train being scanned. There are many variables which effect the ability to measure the bearing temperature rise.
For the sake of safety, hot box detector systems must treat suspicious bearings as overheated bearings. This results in occasional false alarms which, are extremely costly and time consuming.
As a result of the above it is a principal object of the present invention to provide an improved hot box detector system which replaces fixed circuit values with variable values which may be tuned to conditions of the actual train being examined.
A further object is to provide such a system wherein the basic components are compatible with those of existing systems and which may readily be retrofitted into existing systems.
Still further objects and advantages will become apparent from the following description of a preferred embodiment of the invention.